1. Field of the Invention
The present invention relates to a switching power supply which comprises a synchronous rectifier circuit that rectifies A.C. voltage induced in a secondary winding of a transformer.
2. Description of the Prior Art
FIG. 4 is a general circuit diagram showing a forward converter type switching power supply comprising a conventional synchronous rectifier circuit. In this figure, reference numeral 1 denotes a transformer and 2 a switching element such as a MOSFET. A series circuit of a primary winding 1A of the transformer 1 and the switching element 2 is connected across a D.C. power source 3. Reference numeral 4 denotes a synchronous rectifier circuit connected to a secondary winding 1B of the transformer 1. The synchronous rectifying circuit 4 comprises a rectifying MOSFET 5 (Field Effect Transistor) serving as a rectifying element and a freewheeling MOSFET 6 serving as a freewheeling element, both of which turning on or off synchronously with the switching element 2. A series circuit of a choke coil 7 and a capacitor 8 that are used for smoothing purpose is connected across the freewheeling MOSFET 6. A pair of output terminals 9 and 10 for supplying output voltage Vo is connected across the smoothing capacitor 8. In addition, reference numeral 11 denotes a load connected between the output terminals 9 and 10. Reference numerals 15 and 16 are built-in body diodes, which are connected in parallel with reverse polarity between a drain and a source of the rectifying MOSFET 5 and the freewheeling MOSFET 6, respectively.
A pulse width control circuit 12, corresponding to a control circuit for variably controlling pulse conduction width in accordance with the variation of the output voltage Vo, is provided as a feed back circuit for stabilizing the output voltage Vo. The pulse width control circuit 12 supplies a pulse drive signal to the switching element 2 and also supplies the same to the gate of the rectifying MOSFET 5, inverting the pulse drive signal by an inverter 13 and supplying it to the gate of the freewheeling MOSFET 6. In the meantime, reference numeral 14 denotes an insulating transformer for insulating the pulse width control circuit 12 provided in the primary side of the transformer 1 from the rectifying MOSFET 5 and the freewheeling MOSFET 6 provided in the secondary side thereof.
As a result of the switching operation by the switching element 2, a D.C. voltage Vi from the D.C. power source 3 is applied intermittently to the primary winding 1A of the transformer 1 so that an A.C. voltage induced in the secondary winding 2 of the transformer 1 is rectified by the synchronous rectifying circuit 4, which in turn is smoothed by the choke coil 7 and the smoothing capacitor 8, whereby the D.C. output voltage Vo can be provided across the capacitor 8. More specifically, when the switching element 2 turns on by the ON pulse signal from the pulse width control circuit 12, the ON pulse signal is supplied to the gate of the rectifying MOSFET 5 as it is so that the freewheeling MOSFET 6 turns off by an inverted OFF pulse signal while the rectifying MOSFET 5 turns off. Consequently, energy stored in the secondary winding 1B of the transformer 1 is fed to the choke coil 7 and to the load 11 via the rectifying MOSFET 5. Then, the switching element 2 turns off by the OFF pulse signal from the pulse width control circuit 12 so that the freewheeling MOSFET 6 turns on while the rectifying MOSFET 5 turns off. As the result, the energy that has been stored in the choke coil 7 during the ON period of the switching element 2 is fed to the load 11 via the freewheeling MOSFET 6. In the meantime, the smoothing capacitor 8 is provided for absorbing the ripple component of the output voltage Vo.
FIG. 5 shows waveforms in respective elements of the circuit of FIG. 4, in which the uppermost waveform shows voltage Vgs1 between the gate and the source of the switching element 2, and a waveform immediately therebelow shows voltage Vgs2 between the gate and the source of the rectifying MOSFET 5, and then, voltage Vgs3 between the gate and the source of the freewheeling MOSFET 6, choke current IL flowing through the choke coil 7 in a case where the load current has a greater value than critical current Iv, drain current Idq3 flowing through the freewheeling MOSFET 6 in that case, other choke current IL in the case that I load=0, and other drain current Idq3 flowing through the freewheeling MOSFET 6 in that case, respectively.
As shown by the respective waveforms on the left side of FIG. 5, during the operation of the switching element 2, energy is released from the choke coil 7 during the OFF period of the switching element 2 while it is stored in the choke coil 7 during the ON period thereof so that the choke current IL repeatedly fluctuates up and down the level of the load current I load in the same direction under a normal load other than under no-load or light load. The freewheeling MOSFET 6 at this time does not conduct during the ON period of the switching element 2 and thus the drain current Idq3 becomes zero, while it conducts during the OFF period and thus the drain current Idq3 takes a waveform which is the same as that of said choke current IL.
On the other hand, as the load current I load becomes zero in the case of no load, the choke current IL flowing through the choke coil 7 flows in the reverse direction. Accordingly, the drain current Idq3 flowing through the freewheeling MOSFET 6 during the OFF period of the switching element 2 flows from the source to the drain at first but soon decreases linearly until it becomes zero and then flows in the direction from the drain to the source to increase linearly. When the switching element 2 turns on next so that the freewheeling MOSFET 6 turns off, the drain current Idq3 flowing through the freewheeling MOSFET 6 is kept zero until the switching element 2 turns off again. The above-mentioned actions occur not only for no load but also for light load where the load current is under the critical current Iv.
In the case of no load or light load, if, for example, the D.C. input voltage Vi is turned off or an operation stoppage signal is outputted to the pulse width control circuit 12 when an OFF signal is supplied to ON/OFF control terminal(s), the pulse width control circuit 12 interrupts the operation of the switching element 2 immediately. If the operation stoppage signal is outputted when the drain current Idq3 flowing through the freewheeling MOSFET 6 flows in the direction from the source to the drain (at the time P1 in FIG. 5), the voltage Vgs3 between the gate and the source of the freewheeling MOSFET 6 is turned to zero immediately. In this case, surge voltage does not develop between the drain and the source of the freewheeling MOSFET 6 as the current associated with the discharge of energy stored in the choke coil 7 flows through the body diode 16 of the freewheeling MOSFET 6. However, if the operation stoppage signal is outputted when the drain current Idq3 flowing through the freewheeling MOSFET 6 flows in the direction from the drain to the source (at the time P2 in FIG. 5), then not only the freewheeling MOSFET 6 but also the body diode 16 ceases to conduct. As a result, the energy stored in the choke coil 7 loses its way to flow, so that surge voltage develops. For this reason, such surge voltage, though it depends on cases, may become greater than the withstand voltage of the freewheeling MOSFET 6, and thus there have been the possibilities that it may cause the malfunction of the freewheeling MOSFET 6.
To eliminate the above-mentioned problems, it is, therefore, an object of the present invention to provide a switching power supply that can prevent a surge voltage from developing when an operation stoppage signal is outputted at the time of no load or light load.
To attain the object, a switching power supply proposed in the present invention is such that energy is supplied to a smoothing choke coil from a secondary winding of a transformer via a rectifier element when a switching element turns on, while the energy stored in the choke coil is fed to a load through a freewheeling switch element when a switching element turns off, in which at least the freewheeling switch element includes a freewheeling switch element which operates synchronously with the switching element, wherein the switching power supply of the invention includes a unidirectional conductible element which is connected in parallel with reverse polarity across the freewheeling switch element and a surge voltage prevention circuit which detects a timing for the current to flow in a direction in which the unidirectional conductible element conducts when the operation stoppage signal to the switching element is outputted, and then turns off the freewheeling switch element at this timing.
In this case, owing to the surge voltage prevention circuit, even if the operation stoppage signal to the switching element is outputted at any timing, yet the freewheeling switch element turns off at such timing that the current flows in the direction in which the unidirectional conductible element conducts, whereby the current following the discharge of the energy stored in the choke coil is allowed to flow through the unidirectional conductible element.
Preferably, the freewheeling switch element may be a FET, and the unidirectional conductible element may be a body diode of the FET. This way, a body diode of a FET can be used as the unidirectional conductible element.
Also preferably, the surge voltage prevention circuit of the invention may keep the switching element operating until a pulse drive signal to the switching element falls once an operation stoppage signal to the switching element is outputted, whereby the freewheeling switch element can be reliably turned off at any desirable timing.
Still also preferably, the surge voltage prevention circuit of the invention may then detect a falling edge of the pulse drive signal to the switching element to immediately interrupt the supply of the pulse drive signal to the switching element. Thus, the freewheeling switch element can be reliably turned off at such timing that the current flows in the direction in which the unidirectional conductible element conducts.
Alternatively, the present invention may be applied to a device wherein under light load or no load, a current flowing through the freewheeling switch element is turned from the reverse to the forward direction when the freewheeling switch element turns on, and then the freewheeling switch element is turned off when the current flows back to the reverse direction again. In this case, there may desirably be provided a delay circuit for turning off the freewheeling switch element at the timing that the forward current flows through the freewheeling switch element when the surge voltage prevention circuit detects a falling of the pulse drive signal to the switching element. With such delay circuit, the freewheeling switch element can be reliably turned off at desirable timings.